Non-ferrous scrap (except scrap of magnesium and its alloys) which is not too thin, too finely divided or too contaminated with non-metallics typically may be recovered economically and without excessive metal loss by remelting in a reverbatory furnace without the use of salt fluxes. Somewhat finer or moderately contaminated scrap (e.g. relatively thin aluminum siding, mixed low copper clips, some delacquered can scrap, coarse scalper chips) is frequently processed in side-well furnaces in which the scrap is remelted. A burner heats the metal in the main hearth while the scrap is melted by submergence in the side-well without direct flame impingement. Salt fluxes are normally added to promote coalescence.
Due to its extreme reactivity in air, most magnesium scrap (even if heavy and clean) has to be melted under a liquid salt flux or in a protective atmosphere.
However, generally, the thinnest or most finely divided scrap (such as some used beverage cans, borings, turnings, sawings) and drosses or skimmings could not be processed in the above manners without excessive metal loss. Such fine scraps and drosses have normally been melted down in rotary barrel furnaces with the addition of substantial amounts of salt flux to reduce metal loss. Typically, for example in the case of aluminum, the amount of salt flux used is about equal to the weight of non-metallics in the charge of material to be proceesed. Thus, for example, for a 10 ton charge of a dross containing 60% metallic aluminum and 40% non-metallics, the material in the furnace would comprise 6 tons of aluminum, 4 tons of non-metallics, and about 4 tons of salt flux, for a total of about 14 tons. Eventually, 50-55% of the charge might be recovered as molten aluminum (in the example, 5.0-5.5 tons), and about 80-85% of the charge weight (in the example, about 8.0-8.5 tons) might remain as "salt cake". "Salt cake" is a substantial environmental problem and in many areas may no longer be dumped. There is also the significant problem that a substantial amount of salt is vaporized in such processes and then condenses as a fine fume which must be collected in elaborate baghouses. In addition, the salt causes corrosion problems at most stages in the process.
Many attempts have been made to operate conventional rotary barrel furnaces without salt flux. If the charge is dross or fine scrap, the results have generally been unacceptable. For example, as a charge heats up, hot spots can develop and the rate of oxidation can increase rapidly. Within a short while, the heat generation is excessive and temperatures rise rapidly with much loss of metal by oxidation. In extreme cases, outside air is sucked rapidly into the furnace resulting in even more oxidation. Such run-away reactions can produce extremely high, even dangerous, temperature conditions, sometimes well in excess of 1500 C.
Recently, a modified rotary barrel process and apparatus have been developed which avoid the use of salt fluxes in the recovery of non-ferrous metals from dross. In such process, the dross is fed into a rotary barrel furnace, usually but not necessarily tiltable. The dross is then heated by a plasma torch. In the case of aluminum dross, the dross is heated to about 800C., well above the melting point of aluminum at 660C. When this temperature (i.e. 800C.) is reached, the torch is turned off and the furnace flooded with argon. This arrests the oxidation which would otherwise continue. The furnace is rotated for some further period to agglomerate the metal, which is then tapped off or decanted. Finally, the residues are removed from the mouth of the furnace by scraping or tilting.
While the above process has eliminated the serious disadvantages arising from salt fluxes, it too has significant problems. A fundamental deficiency of this process is that it does not allow for optimum control of the process--either during the heating phase or during the agglomeration phase. In particular, the charge is not maintained at the optimum temperature during the final agglomeration phase. In addition, the required use of a plasma torch has some further significant disadvantages. First, it increases the capital cost of the installation very substantially. Second, maintenance of plasma torches is generally more expensive and more complicated than for conventional burner systems. Third, the cost of electricity as an energy source is, in many areas, more than the cost of appropriate fossil fuels such as natural gas or oil. Notwithstanding such deficiencies, it is understood that a plasma torch is required because of its lower gas flow requirements. Published data suggest that, for equal thermal inputs to the furnace, exhaust gas flow from a plasma torch using air in accordance with present commercial practice is about one quarter that from a conventional fuel-air burner. Decreased gas flow is considered desirable because it results in decreased stack losses and a smaller and less complex furnace exhaust gas system.
Most prior art furnaces which are intended to be "closed" have doors which abut up against the lip of the furnace chamber opening. However, uneven build up of residue and damage to the lip frequently prevent such doors from closing effectively. The end result is that such furnaces are virtually open to atmosphere, making them very difficult to operate properly.